Re: [PATCH 8/9] Use reinsert_breakpoint for vCont;s

[Yao Qi <qiyaoltc@gmail.com>]

Pedro Alves <palves@redhat.com> writes:

>> if thread 1 doesn't hit the reinsert breakpoint, we don't have to
>> remove them, because GDB will send vCont;s:1 next time, and GDBserver
>
> There's no guarantee GDB will send vCont;s:1 next time.
> The user may do "continue" instead of "step".
>
>> can only install reinsert breakpoints if they are not installed yet.
>
> The user may even do "return + continue" or "jump", or an infcall,
> all of which resume the thread at a different address from the address
> the thread last stopped.  So there's no guarantee that the
> reinsert breakpoint address makes any sense for the next step request,
> or even that the next resume request is a step in the first place.
>
> Basically the previous step request must be completely forgotten after
> gdb has seen the thread stop.  In all-stop, gdb "sees "all threads
> stopped on each and every event reported to gdb, for any thread.
> A stop reply cancels any and all previous resume requests.

I add some code to delete all reinsert breakpoints for all threads in
all-stop.  See the patch below,

>
>> if thread hits the reinsert breakpoint, but the event is not reported.
>> It becomes pending, and GDBserver will delete the reinsert breakpoints
>> next time when this pending event is reported back to GDB.
>
> I don't follow.  I'm talking about the case where the thread does _not_
> hit the reinsert breakpoint.  Instead some other thread hits some unrelated
> event.

In your review to V2, your words are about other threads hit a breakpoint, but
doesn't mention "thread 1" (requested for resume_step) hits reinsert
breakpoint or not.  I just list two possible results (not hit vs
hit). You didn't talk about the latter, so we don't have to follow it.

-- 
Yao (齐尧)

From f4aa0593190e7e78831532c03177d8264b7dd1c1 Mon Sep 17 00:00:00 2001
From: Yao Qi <yao.qi@linaro.org>
Date: Thu, 19 May 2016 17:07:49 +0100
Subject: [PATCH] Use reinsert_breakpoint for vCont;s

V4: remove all reinsert breakpoints before sending event back to GDB
    in all-stop mode,

V3: install breakpoints in proceed_one_lwp,
    no longer stop all threads when installing breakpoints,
    delete reinsert breakpoints when GDBserver wants to report event,

V2: fix spaces in changelog entry,
    use maybe_hw_step,
    cancel step-over if signal arrives (!maybe_internal_trap),

This patch is to teach GDBserver using software single step to handle
vCont;s.  Simply speaking, if the thread's resume request is resume_step,
install reinsert breakpoint at the next pcs when GDBserver is about to
resume threads.  These reinsert breakpoints of a thread are removed,
when GDBserver gets an event from that thread and reports it back to
GDB.

gdb/gdbserver:

2016-07-05  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (resume_stopped_resumed_lwps): If resume request
	is resume_step, call maybe_hw_step.
	(linux_wait_1): Stop all threads, remove reinsert breakpoints,
	and unstop them.
	(linux_resume_one_lwp_throw): Don't assert the thread has reinsert
	breakpoints or not.
	(proceed_one_lwp): If resume request is resume_step, install
	reinsert breakpoints and call maybe_hw_step.

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index abaf288..b579b4d 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -2563,7 +2563,10 @@ resume_stopped_resumed_lwps (struct inferior_list_entry *entry)
       && !lp->status_pending_p
       && thread->last_status.kind == TARGET_WAITKIND_IGNORE)
     {
-      int step = thread->last_resume_kind == resume_step;
+      int step = 0;
+
+      if (thread->last_resume_kind == resume_step)
+	step = maybe_hw_step (thread);
 
       if (debug_threads)
 	debug_printf ("RSRL: resuming stopped-resumed LWP %s at %s: step=%d\n",
@@ -3622,6 +3625,66 @@ linux_wait_1 (ptid_t ptid,
 
   /* Alright, we're going to report a stop.  */
 
+  /* Remove reinsert breakpoints.  */
+  if (can_software_single_step ())
+    {
+      /* Remove reinsert breakpoints or not.  It it is true, stop all
+	 lwps, so that other threads won't hit the breakpoint in the
+	 staled memory.  */
+      int remove_reinsert_breakpoints_p = 0;
+
+      if (non_stop)
+	{
+	  remove_reinsert_breakpoints_p
+	    = has_reinsert_breakpoints (current_thread);
+	}
+      else
+	{
+	  /* In all-stop, a stop reply cancels all previous resume
+	     requests.  Delete all reinsert breakpoints.  */
+	  struct inferior_list_entry *inf, *tmp;
+
+	  ALL_INFERIORS (&all_threads, inf, tmp)
+	    {
+	      struct thread_info *thread = (struct thread_info *) inf;
+
+	      if (has_reinsert_breakpoints (thread))
+		{
+		  remove_reinsert_breakpoints_p = 1;
+		  break;
+		}
+	    }
+	}
+
+      if (remove_reinsert_breakpoints_p)
+	{
+	  /* If we remove reinsert breakpoints from memory, stop all lwps,
+	     so that other threads won't hit the breakpoint in the staled
+	     memory.  */
+	  stop_all_lwps (0, event_child);
+
+	  if (non_stop)
+	    {
+	      gdb_assert (has_reinsert_breakpoints (current_thread));
+	      delete_reinsert_breakpoints (current_thread);
+	    }
+	  else
+	    {
+	      struct inferior_list_entry *inf, *tmp;
+
+	      ALL_INFERIORS (&all_threads, inf, tmp)
+		{
+		  struct thread_info *thread = (struct thread_info *) inf;
+
+		  if (has_reinsert_breakpoints (thread))
+		    delete_reinsert_breakpoints (thread);
+		}
+	    }
+
+	  unstop_all_lwps (0, event_child);
+	}
+    }
+
   if (!stabilizing_threads)
     {
       /* In all-stop, stop all threads.  */
@@ -4275,12 +4338,6 @@ linux_resume_one_lwp_throw (struct lwp_info *lwp,
 
       step = maybe_hw_step (thread);
     }
-  else
-    {
-      /* If the thread isn't doing step-over, there shouldn't be any
-	 reinsert breakpoints.  */
-      gdb_assert (!has_reinsert_breakpoints (thread));
-    }
 
   if (fast_tp_collecting == 1)
     {
@@ -5088,7 +5145,14 @@ proceed_one_lwp (struct inferior_list_entry *entry, void *except)
       if (debug_threads)
 	debug_printf ("   stepping LWP %ld, client wants it stepping\n",
 		      lwpid_of (thread));
-      step = 1;
+
+      /* If resume_step is requested by GDB, install reinsert
+	 breakpoints when the thread is about to be actually resumed if
+	 the reinsert breakpoints weren't removed.  */
+      if (can_software_single_step () && !has_reinsert_breakpoints (thread))
+	install_software_single_step_breakpoints (lwp);
+
+      step = maybe_hw_step (thread);
     }
   else if (lwp->bp_reinsert != 0)
     {